The present invention relates generally to the field of communication, and, more particularly, to communication transmitters.
To transmit digital data over a wireless or public switched telephone network (PSTN), modulation may be used to code the digital information into analog signals. Common modulation techniques modulate digital information into two orthogonal signal components, referred to as the in-phase component and the quadrature component. Thus, digital information may be represented by a signal comprising a combination of the in-phase and quadrature components.
The signal set representing the digital information is commonly displayed in a two-dimensional signal space or constellation diagram in which the number of points in the constellation is given by 2n, where n is the number of bits to be encoded. For example, FIG. 1 illustrates an eight-symbol constellation diagram where the horizontal axis represents the in-phase component, the vertical axis represents the quadrature component, and each symbol represents three bits of data. Note that the three bits of data represented by each point in the constellation may be provided from the output of an encoder, such as a convolutional encoder, that operates on the bits of a message signal to be transmitted.
A received symbol may not correspond precisely to the ideal symbol shown in the constellation diagram due to noise associated with the communication channel and imperfections of both the transmitter and receiver. As shown in FIG. 1, the difference between a received symbol (shaded constellation point) and an ideal symbol (solid constellation point) can be represented as an error vector. Generally, the smaller the magnitude of the error vector, the better the performance of the communication system. Error vector magnitude is the root mean square (RMS) value of the error vector over time at the precise time instance of the symbol clock transitions. EVM is typically normalized to either the amplitude of the outermost symbol, or the square root of the average symbol power. Each symbol may be represented as a particular amplitude A and phase θ. Thus, the transmitted signal may vary in amplitude and/or phase to transmit a string of consecutive symbols. The amplitude and phase components of a signal may be processed separately in a transmitter. It has been found that the EVM for a communication system may vary based on the delay applied between the amplitude A of a transmitted signal and the phase θ of the transmitted signal. Thus, a transmitter may be programmed to operate using a delay that on average provides acceptable EVM performance.
Another performance criterion that may be used to evaluate a communication system is adjacent channel power. A transmitter is typically designed to focus its transmission power in a limited bandwidth that corresponds to the communication channel currently used by the transmitter. Unfortunately, a typical transmitter does not achieve zero power spectral density outside of its current communication channel. The signal transmission power that is measured at frequencies adjacent the communication channel bandwidth when the transmitter transmits is generally known as adjacent channel power (ACP). The ratio of ACP with the power within the main channel bandwidth is defined as adjacent channel power ratio (ACPR) and is the definitive measure of spectral re-growth due to transmitter non-linearity. These signals outside of the bandwidth used by the transmitter for communication may interfere with other communication sessions operating on those frequencies. Accordingly, it is generally desirable to reduce the adjacent channel power ratio to an acceptably low level. Like EVM, it has been found that the ACPR for a communication system may vary based on the delay applied between the amplitude A of a transmitted signal and the phase θ of the transmitted signal. Thus, a transmitter may be programmed to operate using a delay that on average provides acceptable ACPR performance.
Unfortunately, a delay that provides acceptable ACPR performance may not necessarily provide acceptable EVM performance and vice versa. Moreover, a delay that provides acceptable ACPR and/or EVM performance at one transmit power level may not necessarily provide acceptable ACPR and/or EVM performance at another transmit power level.